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<h4 class="subsection">14.21.4 RTL SSA Resources</h4>

<p>The RTL SSA form tracks two types of &ldquo;resource&rdquo;: registers and memory.
Each hard and pseudo register is a separate resource.  Memory is a
single unified resource, like it is in GIMPLE (see <a href="GIMPLE.html#GIMPLE">GIMPLE</a>).
</p>
<p>Each resource has a unique identifier.  The unique identifier for a
register is simply its register number.  The unique identifier for
memory is a special register number called <code>MEM_REGNO</code>.
</p>
<p>Since resource numbers so closely match register numbers, it is sometimes
convenient to refer to them simply as register numbers, or &ldquo;regnos&rdquo;
for short.  However, the RTL SSA form also provides an abstraction
of resources in the form of <code>rtl_ssa::resource_info</code>.
This is a lightweight class that records both the regno of a resource
and the <code>machine_mode</code> that the resource has (see <a href="Machine-Modes.html#Machine-Modes">Machine Modes</a>).
It has functions for testing whether a resource is a register or memory.
In principle it could be extended to other kinds of resource in future.
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